The Research Plan is divided into four tightly inter-linked projects that will test novel hypotheses regarding the central neural circuitry that 1) affects basal sympathetic nerve discharge (SND) and the respiratory rhythm as reflected by phrenic nerve activity (PNA) and 2) mediates changes in SND, blood pressure (BP), and respiration produced by activation of an array of vagal and sympathetic afferents. These projects build on work completed during the current funding period and address problems from a unique perspective by focusing on the often ignored "classic pressor area" in the medullary lateral tegmental field (LTF). The experiments described in Project #1 are the first to seek to identify the source(s) of excitatory drive to LTFsympathoexcitatory neurons that help set the basal level of SND. These experiments have significant bearing on the mechanism by which the neurogenic support for resting BP is generated. Projects #2 & 3 provide the first comprehensive assessment of mechanisms by which "natural" activation of different types of visceral receptors (mechanoreceptors, chemosensitive receptors) can alter efferent SND, PNA, and ultimately cardiorespiratory function. The reflexes involve activation of myelinated and unmyelinated fibers from cardiopulmonary, gastrointestinal, and renal receptors, and responses include both excitation and inhibition of SND and/or PNA. These reflexes are thought to contribute to cardiorespiratory changes noted during physiological (e.g., lung inflation, digestion, fluid balance) as well as pathophysiological states (e.g., congestive heart failure, myocardial ischemia, sudden infant death syndrome, and esophageal dysfunction). Project #4 addresses the previously unrecognized role of the LTF in determining central respiratory rate. These studies are expected to show that medullary circuits regulating SND are more complexly organized than depicted in most current reviews and that the LTF plays a hitherto unrecognized role in regulating the respiratory rhythm. [unreadable] [unreadable]